The present invention is directed toward the recovery of hydrocarbons heavier than methane from liquefied natural gas (LNG) and in particular to an improved process that uses a portion of the LNG as reflux in the separation process to aid in the recovery of the heavier than methane hydrocarbons.
Natural gas typically contains up to 15 vol. % of hydrocarbons heavier than methane. Thus, natural gas is typically separated to provide a pipeline quality gaseous fraction and a less volatile liquid hydrocarbon fraction. These valuable natural gas liquids (NGL) are comprised of ethane, propane, butane, and minor amounts of other heavy hydrocarbons. In some circumstances, as an alternative to transportation in pipelines, natural gas at remote locations is liquefied and transported in special LNG tankers to appropriate LNG handling and storage terminals. The LNG can then be revaporized and used as a gaseous fuel in the same fashion as natural gas. Because the LNG is comprised of at least 80 mole percent methane it is often necessary to separate the methane from the heavier natural gas hydrocarbons to conform to pipeline specifications for heating value. In addition, it is desirable to recover the NGL because its components have a higher value as liquid products, where they are used as petrochemical feedstocks, compared to their value as fuel gas.
NGL is typically recovered from natural gas streams by many well-known processes including xe2x80x9clean oilxe2x80x9d adsorption, refrigerated xe2x80x9clean oilxe2x80x9d absorption, and condensation at cryogenic temperatures. Although there are many known processes, there is always a compromise between high recovery and process simplicity (i.e., low capital investment). The most common process for recovering NGL from LNG is to pump and vaporize the LNG, and then redirect the resultant gaseous fluid to a typical industry standard turbo-expansion type cyrogenic NGL recovery process. Such a process requires a large pressure drop across the turbo-expander or J.T. valve to generate cryogenic temperatures. In addition, such prior processes typically require that the resultant gaseous fluid, after LPG extraction, be compressed to attain the preexpansion step pressure. Alternatives to this standard process are known and two such processes are disclosed in U.S. Pat. Nos. 5,588,308 and 5,114,451. The NGL recovery process described in the ""308 patent uses autorefrigeration and integrated heat exchange instead of external refrigeration or feed turbo-expanders. This process, however, requires that the LNG feed be at ambient temperature and be pretreated to remove water, acid gases and other impurities. The process described in the ""457 patent recovers NGL from a LNG feed that has been warmed by heat exchange with a compressed recycle portion of the fractionation overhead. The balance of the overhead, comprised of methane-rich residual gas, is compressed and heated for introduction into pipeline distribution systems.
Our invention provides another alternative NGL recovery process that produces a low-pressure, liquid methane-rich stream that can be directed to the main LNG export pumps where it can be pumped to pipeline pressures and eventually routed to the main LNG vaporizers. Moreover, our invention uses a portion of the LNG feed directly as an external reflux in the separation process to achieve high yields of NGL as described in the specification below and defined in the claims which follow.
As stated, our invention is directed to an improved process for the recovery of NGL from LNG which avoids the need for dehydration, the removal of acid gases and other impurities. A further advantage of our process is that it significantly reduces the overall energy and fuel requirements because the residue gas compression requirements associated with a typical NGL recovery facility are virtually eliminated. Our process also does not require a large pressure drop across a turbo-expander or J.T. value to generate cryogenic temperatures. This reduces the capital investment to construct our process by 30 to 50% compared to a typical cryogenic NGL recovery facility.
In general, our process recovers hydrocarbons heavier than methane using low pressure liquefied natural gas (for example, directly from an LNG storage system) by using a portion of the LNG feed, without heating or other treatment, as an external reflux during the separation of the methane-rich stream from the heavier hydrocarbon liquids, thus producing high yields of NGL. The methane-rich stream from the separation step is routed to the suction side of a low temperature, low head compressor to re-liquefy the methane rich stream. This re-liquefied LNG is then directed to main LNG export pumps. In alternative flow schemes, as presented below, compression of the methane rich stream is unnecessary when high pressure LNG is used in heat exchange with stabilizer overhead and pumps are used on the recovery bottoms bypassed feed stream.
In an alternate version of our process, the low pressure liquid LNG feed is spilt twice to supply two external reflux streams to two separation columns (for example, a cold separator and a stabilizer). The overhead from each of these towers is combined to form a methane rich stream substantially free of NGL. Possible variations of our process include recovering substantially all of the ethane and heavier hydrocarbons from the LNG, rejecting the ethane while recovering the propane and heavier hydrocarbons, or similarly performing this split of any desired molecular weight hydrocarbon. Also, boil-off vapor can be added to the methane rich stream prior to heat exchange with the incoming low pressure liquid LNG. Boil-off vapor is typically obtained from LNG storage tanks as waste or escaped vapor. In one of the possible variations of our process, ethane recoveries are in the range of about 91 to 95% with 99% propaneplus recovery. In another variation, a typical propane recovery in the ethane rejection mode of operation is from about 94 to about 96% with 99% butane-plus recovery. Similarly, propane could be left in the gaseous stream while recovering 94 to 96% of the butanes.